The Insulated-Gate Bipolar Transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch and in newer devices is noted for combining high efficiency and fast switching. It switches electric power in many modern appliances such as: Variable-Frequency Drives (VFDs), electric cars, trains, variable speed refrigerators, lamp ballasts, air-conditioners, and even stereo systems with switching amplifiers.
IGBTs are often used for high voltage (e.g., greater than 600V) and high-current power converter applications. During normal switching, in order to achieve overall system efficiency, one has to keep the IGBT high current/high voltage switching energy loss to a minimum and the Miller Plateau short. To achieve these goals, one has to drive the IGBT hard, using a small gate resistor and/or external buffer circuit. While doing this, the peak collector-to-emitter voltage of the IGBT may exceed its absolute maximum rating. This means that a protection circuit is required to protect the IGBT from damage.
Furthermore, during short-circuit conditions, for example a short circuit of the load wire to the power source, large current flows through the IGBT, thereby damaging the IGBT. Gate drive circuits are used to detect IGBT short circuit conditions and trigger the turn-off of the IGBT safely to prevent further damage to the IGBT.
In order to keep the collector-to-emitter voltage of the IGBT less than its absolute maximum rating during high-current turn-off, one straight forward solution is to slow down the turn-off using a relatively large resistor. A common IGBT drive circuit 100 without short-circuit protection is shown in FIG. 1 where a load 112 is driven by current from IGBTs 108, which are in turn driven by gate drivers 104. The depicted circuit 100 is often referred to as a half-bridge circuit and is among the most important circuit configurations for power drives. The circuit 100 is shown to include two IGBTs 108 connected to one another at the circuit's 100 midpoint and the load 112 is connected to this midpoint. The midpoint corresponds to a circuit node where an emitter E of one IGBT 108 is connected to a collector C of another IGBT 108.
Problematically, as shown in FIG. 1, if the circuit 100 experiences a short (e.g., between Ground/common voltage and the circuit's 100 midpoint) then excessive current will flow through the top IGBT 108, most likely resulting in damage to the IGBT 108. As shown in FIG. 1, the first circuit 100 is shown to include resistor Rg_off, which is used to slow down the turn-off time for the IGBT 108. However, the problem with slowing down the turn-off time is the energy losses are increased and the Miller Plateau is lengthened, thereby reducing the overall efficiency of the system. Furthermore, by increasing the turn-off time, there is an increased likelihood that short-circuit conditions may occur for a longer amount of time before they can be detected, thereby resulting in catastrophic damage to the IGBT.
To address the problems of circuit 100, (e.g., high energy loss, long Miller Plateau, etc.), one can use a fast shutdown with the help of a protection circuit. Fast shutdown can help to keep the turn-off energy losses to a minimum, shorten the Miller Plateau, and meet shutdown system dead time requirements. However, turning off the IGBT too fast during high current conditions will result in high peak collector-to-emitter voltage of the IGBT 108, which could damage the IGBT 108.
FIG. 2 depicts another circuit 200 that includes a protection circuit as described above. In particular, circuit 200 is shown to include an additional Transient Voltage Suppressor (TVS) 204 or clamping diode positioned between the collector C and gate G of the IGBT, to prevent IGBT breakdown. When the collector-to-emitter voltage of the IGBT 108 exceeds the TVS 204 breakdown voltage, the IGBT is turned on again to clamp the collector-to-emitter voltage.
While the TVS 204 works well to protect the IGBT 108 from collector-to-emitter overvoltage, the implementation of the TVS 204 reduces the effective working voltage of the IGBT 108. For the same application, higher voltage IGBTs have to be used, which increases the overall system costs and reduces the system efficiency.